Field
The present disclosure relates generally to energy storage devices, and more particularly to metal and metal-ion battery technology and the like.
Background
Owing in part to their relatively high energy densities, light weight, and potential for long lifetimes, advanced metal-ion batteries such as lithium-ion (Li-ion) batteries are desirable for a wide range of consumer electronics. However, despite their increasing commercial prevalence, further development of these batteries is needed, particularly for potential applications in low- or zero-emission, hybrid-electrical or fully-electrical vehicles, consumer electronics, energy-efficient cargo ships and locomotives, aerospace applications, and power grids.
Conversion-type electrodes, such as fluorides, sulfides, oxides, nitrides, phosphides, hydrides and others for Li-ion batteries offer high gravimetric and volumetric capacities.
Fluorides, in particular, offer a combination of relatively high average voltage and high capacities, but suffer from several limitations for various metal-ion (such as Li-ion) battery chemistries. For example, only selected metal fluoride particles have been reported to offer some reasonable cycle stability in Li-ion battery cells (specifically AgF2, FeF2, FeF3, CoF2, and NiF2). Many other metal fluorides are commonly believed not to be practical for applications in Li-ion batteries due to the irreversible changes that occur in such cathodes during battery operation. For example, during Li-ion insertion into some of the other fluorides (such as CuF2, for example) and the subsequent formation of LiF during the conversion reaction, the original fluoride-forming element (such as Cu in the case of CuF2) produces electrically isolated (Cu) nanoparticles. Being electrically isolated, such nanoparticles cannot electrochemically react with LiF to transform back into CuF2 during subsequent Li extraction, thereby preventing reversibility of the conversion reaction. As a result, after a discharge, the cell cannot be charged back to the initial capacity.
However, even the cathodes based on those metal fluorides that are believed to be most practical due to their relatively reversible operation and reasonably low cost (such as FeF2, FeF3, CoF2, and NiF2), suffer from multiple limitations including: (i) low electrical conductivity, which limits their utilization and both energy and power characteristics in batteries; (ii) low ionic conductivity, which limits their utilization and both energy and power characteristics in batteries; and (iii) volume changes during metal ion insertion/extraction, which may cause mechanical and electrical degradation in the electrodes during battery operation. As a result, despite the theoretical advantages of fluoride-based cathodes, for example, their practical applications in metal-ion batteries are difficult to achieve. The cells produced with fluoride-based cathodes currently suffer from poor stability, volume changes, slow charging, and high impedance.
Several approaches have been developed to overcome some of the above-described difficulties, but none have been fully successful in overcoming all of them.
For example, decreasing particle size decreases the ion diffusion distance, and offers one approach to addressing the low ionic conductivity limitation. However, nanopowders suffer from high electrical resistance caused by the multiple, highly resistive point contacts formed between the individual particles. In addition, small particle size increases the specific surface area available for undesirable electrochemical side reactions. Furthermore, simply decreasing the particle size does not address and may in some cases exacerbate other limitations of such materials, such as volume changes as well as weakening of the particle-binder interfaces. Finally, in contrast to using micron-scale particles for cathode formulations, handling nanoparticles and using them to prepare dense electrodes is technologically difficult. Nanoparticles are difficult to disperse uniformly within conductive carbon additives and binder of the cathode and the undesirable formation of agglomerates of nanoparticles tends to take place. Formation of such agglomerates reduces the electrode density (thus reducing volume-normalized capacity and energy density of the cells), reduces electrode stability (since binder and conductive additives do not connect individual particles within such agglomerates) and reduces capacity utilization (since some of the nanoparticles become electrically insulated and thus do not participate in Li-ion storage).
In another approach, selected metal fluoride particles which offer some reasonable cycle stability in Li-ion battery cells (specifically FeF2, FeF3, CoF2, and NiF2) may be mechanically mixed with (in some cases by using high energy milling, as described, for example, in U.S. Pat. No. 7,625,671 B2) or deposited onto the surface of conductive substrates, such as carbon black, graphite, multi-walled carbon nanotubes, or carbon fibers. In this case, the high electrical conductivity of the carbon enhances electrical conductivity of the electrodes. However, the phase transformations during battery operation and the volume changes discussed above may induce the separation of the active material from the conductive additives, leading to resistance growth and battery degradation.
In yet another approach, selected metal fluoride particles (specifically FeF2 particles) may be coated with a solid multi-walled graphitic carbon shell layer. In this case, the electrical conductivity of a metal fluoride cathode may be improved. However, the above-described volume changes during metal ion insertion may break the graphitic carbon coating and induce irreversible capacity losses. Similarly, the phase transformation during subsequent charging and discharging cycles may induce a separation of the active material from the graphitic carbon shell, leading to resistance growth and battery degradation.
Accordingly, there remains a need for improved metal and metal-ion batteries, components, and other related materials and manufacturing processes.